The present invention relates to an exposure tool and method for use in a process for manufacturing a semiconductor device, such as an LSI, and, in particular, to a technique for improving the overlay accuracy of a pattern projected by an optical type stepper and scanner.
Recently, with a microminiaturization of a circuit pattern of an LSI (large scale integrated circuit), an optical stepper of a high resolution type has been widely used as a pattern transfer means. When a device pattern is written on a wafer with the use of this exposure tool, it is necessary to highly accurately align a semiconductor wafer prior to being exposed with light. Upon alignment, a mark position is normally detected on a wafer. The method for detecting the position of the wafer mark is largely classified into a die-by-die system and global alignment system. The die-by-die system detects a mark position on a wafer at each chip for alignment and can achieve high overlay accuracy. The global alignment system, on the other hand, detects several alignment marks on a wafer with an alignment optical system and exposes the wafer with light by correcting a chip array from those detected mark positions. The die-by-die system can achieve high overlay accuracy, but is poor in productivity. This is the reason why the global alignment system is increasingly used.
FIG. 1 diagrammatically shows an arrangement mechanism in the conventional stepper and an alignment signal detecting system. FIG. 2 shows one practical array form of alignment marks 9 initially formed on a semiconductor wafer 4 in FIG. 1. In FIG. 1, a wafer 4 is placed on a wafer stage 5 and the wafer stage 5 can be translated by a wafer stage drive controller, not shown, in an xy direction to the wafer stage 5. A reticle having a light exposure area with a circuit pattern written thereon by an electronic beam is placed on a reticle stage 10 and the reticle stage 10 is translated by a reticle stage drive controller, not shown, in an xy direction to the reticle stage.
The alignment mechanism 1 is arranged near a projection lens 2 and illuminates the alignment marks 9 on the wafer 4, by alignment light, such as an HeNe laser 3, from an alignment light illuminating section, not shown, and allows the alignment light which is reflected/diffracted on the alignment marks 9 to be converted by a light receiving mechanism, not shown, to an electric signal and then by an alignment signal processing circuit (not shown) to position information (alignment output signal). A wafer stage laser interferometer 7 illuminates, with laser light, a mirror 7a mounted on the wafer stage 5 and measures the position of the wafer stage 5 by detecting the laser beam reflected on the mirror. A calculation device 8 calculates the alignment mark position information from the alignment mechanism 1 and wafer stage position information from the laser interferometer 7 and outputs, based on a result of calculation, a signal for controlling the position of the wafer stage 5.
Explanation will be given about the light exposure method using the light exposure tool.
Prior to exposing the wafer 4 with light, the alignment mark position is measured by the alignment optical system with the wafer 4 mounted on the wafer stage 5 and a shot array on a wafer surface is found from the alignment mark measuring position. The shot array position on the wafer surface can be separated into a linear error Lw, that is, a linear systematic error, and random error (Swx, Swy) in an interfield of the wafer.
The wafer's linear error Lw contains, as shown in FIG. 3A, a translate error (.alpha.x, .alpha.y) in an xy direction, rotation error (an error in a rotation direction of the wafer) .theta.w, a scaling error (Ex, Ey), that is, an error representing an expansion/contraction error, and an orthogonality error .theta.wo in the interfield.
With (dxw, dyx) representing the difference between a designed mark position (x, y) and a mark position obtained by measurement, the mark detection position error on the wafer can be expressed as the functions of the coordinate (x, y) in the interfield given by EQU dxw=.alpha.x-(.theta.w+.theta.wo).multidot.y+Ex.multidot.x+Swx EQU dyw=.alpha.y+.theta.w.multidot.x+Ey.multidot.y+Swy (1)
where
(.alpha.x, .alpha.y): the translate error coefficient in the xy direction; PA1 .theta.w: the rotation error coefficient; PA1 (Ex, Ey): the scaling error coefficient representing the expansion/contraction of the wafer; PA1 .theta.wo: the orthogonality error coefficient; and PA1 (Swx, Swy): the remaining random error coefficient. PA1 .DELTA.xw=.alpha.x-(.theta.w+.theta.wo).multidot.y+Ex.multidot.x PA1 .DELTA.yw=.alpha.y+.theta.w.multidot.x+Ey.multidot.y PA1 (Mx, My): the magnification error coefficient representing an expansion/contraction in the shot; PA1 .theta.s: the shot rotation error coefficient; PA1 .theta.so: the shot skew error coefficient; and PA1 (Ssx, Ssy): the remaining random error coefficient. PA1 a wafer stage having a semiconductor wafer with a plurality of alignment marks formed thereon for position identification; PA1 a wafer stage position measuring mechanism for measuring a position of the wafer stage; PA1 a projection optical mechanism for projecting a circuit pattern onto the wafer to expose a predetermined area of the wafer with beam; PA1 an alignment mechanism for detecting the positions of wafer in an aligned state; PA1 a calculation mechanism for calculation-processing a signal obtained by processing an alignment output signal from the alignment mechanism and output signals obtained from the wafer stage position measuring mechanism; and PA1 a control mechanism which, at a plurality of exposures of the wafer by the projection optical mechanism, controls the alignment mechanism in accordance with an n-th order function (x, y) in an interfield coordinate (x, y) of the wafer by approximating a per-exposure systematic error of at least one of a shot rotation error, magnification error and skew error with the n-th order function Ls (x, y) and corrects the per-exposure systematic error depending upon the interfield coordinate. PA1 effecting calculation-processing by finding at least one of systematic errors Ls (.theta.s, Mx, My, .theta.so) at each of a plurality of shots created on the wafer and approximating at least one of a shot rotation error, magnification error and skew error with an n-th order function Ls (x, y) in an interfield coordinate (x, y) of the wafer; and PA1 effecting control by correcting at least one of a shot rotation (.theta.s), shot magnification (Mx, My) and shot skew (.theta.so), during the writing of the circuit pattern on the wafer, in accordance with the function Ls (x, y).
These linear error coefficients (.alpha.x, .alpha.y, Ex, Ey, .theta.w, .theta.wo) in the interfield of the wafer is found by the calculation device 8 using the least squares method.
The shot position (xw, yw) actually exposed with the light after alignment is given by the following expressions (2) with respect to a designed value (X, Y): EQU xw=X+.DELTA.xw EQU yw=Y+.DELTA.yw (2)
where
On the other hand, the intrafield error of the shot can be separated, as shown in FIG. 3B, into an intrafield linear error Ls (shot rotation, magnification and skew errors) and intrafield random error (Ssx, Ssy). With (dxs, dys) representing the difference between the designed mark position (Xs, Ys) and the mark position obtained by measurement, the position error in the intrafield can be expressed as the functions in the intrafield coordinate (xs, ys) which can be given by: EQU dxs=-(.theta.s+.theta.so).multidot.ys+Mx.multidot.xs+Ssx EQU dys=+.theta.s.multidot.xs+My.multidot.ys+Ssy (3)
where
These linear error coefficients (Mx, My, .theta.s, .theta.so) in the intrafield are found with the use of the least squares method.
In a stepper type light exposure tool of a step-and-repeat light exposure type, it is possible to correct the shot rotation (.theta.s) and isotropic magnification (Mx+My)/2 of the intrafield linear errors Ls.
In the scan type light exposure tool, on the other hand, using a mercury-vapor lamp or other light sources such as an excimer laser and adapted to expose the wafer with the light by moving the wafer and reticle relative to each other while moving the wafer stage and reticle stage, it is possible to, while under the scan light exposure, correct the linear errors Ls (.theta.s, Mx, My, .theta.so) in the intrafield, individually.
Conventionally, the correction of the intrafield linear error Ls has been made by correcting the constant. There occurs a variation in the intrafield systematic errors, depending upon the wafer stage position, such as an error resulting from a variation in the shot rotation by a wafer stage position displacement resulting from a yawing error of the wafer stage 5 and mirror bending (distortion) of the laser interferometer 7 serving as the wafer stage position reference. The correction of such an error resulting from such a variation has not been done in the current practice. With the recent increase in the size of a wafer, such intrafield systematic error varying depending upon the position displacement of the wafer stage has reached a stage where it cannot be disregarded any longer. As set out above, such intrafield systematic error Ls has not been corrected in any conventional light exposure tool and method using the conventional optical stepper, surely presenting a near-future problem.